As known in the art, control procedures applied to an internal combustion engine mounted, for example, in a vehicle, include air-fuel ratio control for controlling the air-fuel ratio of air-fuel mixture to be combusted in a cylinder. In the air-fuel ratio control, the air-fuel ratio of combusted air-fuel mixture is obtained from a detection result of the oxygen concentration in the exhaust gas, and a fuel injection amount is feedback-controlled such that the obtained air-fuel ratio becomes a target value.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-048711. Conventionally, Patent Document 1 has disclosed one example of a control device for an internal combustion engine that executes air-fuel ratio control. The device disclosed in the document has a main air-fuel ratio sensor located upstream of a catalyst in the exhaust pipe and a sub-oxygen sensor located downstream of the catalyst. Main feedback control is executed in which output of the main air-fuel ratio sensor is fed back to the fuel injection amount such that the air-fuel ratio at a section upstream of the catalyst becomes equal to a target air-fuel ratio. Concurrently, sub-feedback control is executed in which output of the sub-oxygen sensor is fed back to the fuel injection amount such that the air-fuel ratio of the exhaust gas flowing out of the catalyst becomes equal to the stoichiometric air-fuel ratio.
As described in the publication, the range of air-fuel ratio that allows a catalyst to properly purify exhaust gas, which is referred to as a window, changes in accordance with changes in the operating state of an internal combustion engine. For example, it is known that during a high load operation of an internal combustion engine, an air-fuel ratio optimal for exhaust purification becomes richer. The reason for this is as follows. That is, during a high load operation, the amount of oxygen entering a catalyst per unit time increases. Storing of oxygen by a catalyst is a physical phenomenon and therefore occurs very rapidly. In contrast, release of oxygen from the catalyst is a chemical reaction and therefore occurs relatively slowly. Thus, when a large amount of oxygen flows into the catalyst, the oxygen concentration is increased in the catalyst. This shifts the window to the richer side.